Inherent Stiffness Research Articles

Synthesis of auxetic structures using optimization of compliant mechanisms and a micropolar material model

Aim of this work is the synthesis of auxetic structures using a topology optimization approach for micropolar (or Cosserat) materials. A distributed compliant mechanism design problem is formulated, adopting a SIMP---like model to approximate the constitutive parameters of 2D micropolar bodies. The robustness of the proposed approach is assessed through numerical examples concerning the optimal design of structures that can expand perpendicularly to an applied tensile stress. The influence of the material characteristic length on the optimal layouts is investigated. Depending on the inherent flexural stiffness of micropolar solids, truss---like solutions typical of Cauchy solids are replaced by curved beam---like material distributions. No homogenization technique is implemented, since the proposed design approach applies to elements made of microstructured material with prescribed properties and not to the material itself.

A correlation method for high-frequency response of a cargo during dry transport in high seas

Cargo, such as a Tension Leg Platform (TLP), Semi-submersible platform (Semi), Spar or a circular Floating Production Storage and Offloading (FPSO), are frequently dry-transported on a Heavy Lift Vessel (HLV) from the point of construction to the point of installation. The voyage can span months and the overhanging portions of the hull can be subject to frequent wave slamming events in rough weather. Tie-downs or sea-fastening are usually provided to ensure the safety of the cargo during the voyage and to keep the extreme responses of the cargo, primarily for the installed equipment and facilities, within the design limits. The proper design of the tie-down is dependent on the accurate prediction of the wave slamming loads the cargo will experience during the voyage. This is a difficult task and model testing is a widely accepted and adopted method to obtain reliable sea-fastening loads and extreme accelerations. However, it is crucial to realize the difference in the inherent stiffness of the instrument that is used to measure the tri-axial sea fastening loads and the prototype design of the tie-downs. It is practically not possible to scale the tri-axial load measuring instrument stiffness to reflect the real tie-down stiffness during tests. A correlation method is required to systematically and consistently account for the stiffness differences and correct the measured results. Direct application of the measured load tends to be conservative and lead to over-design that can reflect on the overall cost and schedule of the project. The objective here is to employ the established correlation method to provide proper high-frequency responses to topsides and hull design teams. In addition, guidance for optimizing tie-down design to avoid damage to the installed equipment, facilities and structural members can be provided.

A direct correlation of x-ray diffraction orientation distributions to the in-plane stiffness of semi-crystalline organic semiconducting films

Large charge mobilities of semi-crystalline organic semiconducting films could be obtained by mechanically aligning the material phases of the film with the loading axis. A key element is to utilize the inherent stiffness of the material for optimal or desired alignment. However, experimentally determining the moduli of semi-crystalline organic thin films for different loading directions is difficult, if not impossible, due to film thickness and material anisotropy. In this paper, we address these challenges by presenting an approach based on combining a composite mechanics stiffness orientation formulation with a Gaussian statistical distribution to directly estimate the in-plane stiffness (transverse isotropy) of aligned semi-crystalline polymer films based on crystalline orientation distributions obtained by X-ray diffraction experimentally at different applied strains. Our predicted results indicate that the in-plane stiffness of an annealing film was initially isotropic, and then it evolved to transverse isotropy with increasing mechanical strains. This study underscores the significance of accounting for the crystalline orientation distributions of the film to obtain an accurate understanding and prediction of the elastic anisotropy of semi-crystalline polymer films.

The long-term behaviour of retaining walls in Dublin

The performance of retaining walls in Dublin Boulder Clay under undrained conditions is relatively well understood. These structures show very modest lateral movements; this is attributed to the inherent strength and stiffness of the boulder clay and the very slow dissipation of excavation-induced pore water suctions. There are few data available on the long-term behaviour of such walls, however, both in Dublin and elsewhere. The economic downturn in Ireland provided a unique opportunity to improve understanding of the long-term behaviour of these types of excavations. Several temporary works retaining walls remained in place for long periods due to work on the projects having been suspended. The paper gives details of several such projects but focuses on the performance of the 14 m high secant piled wall with a single row of ground anchors at Heuston South Quarter. It was designed as temporary works but remained in place for about 7 years, with movements increasing to over 50% more than those predicted at design stage. The paper will present a detailed account of the Heuston wall design, construction and performance and the subsequent remedial works, which were carried out to ensure its long-term stability.

Intrinsic correlation between elastic modulus and atomic bond stiffness in metallic glasses

ZrxCu100−x (x=15–70) metallic glass (MG) models were constructed by molecular dynamics simulations to explore the intrinsic correlation between elastic modulus and atomic bond stiffness in MGs. The elastic modulus and atomic mole fraction of MGs approximately behave parabolically, and their maxima location shifts to lower Zr mole fraction. This phenomenon is due to the variation of their atomic bond percentages, which suggests that elastic modulus reflects the inherent stiffness of atomic bonds. The “rule of mixture”, which uses atomic bonds as the components, is more applicable than that in which atoms are used as the components to explain the inheritance of elastic modulus.

Ultrasonic measurements of attenuation and velocity of compressional and shear waves in partially frozen unconsolidated sediment and synthetic porous rock

The presence of partially frozen liquid in the pore spaces of porous materials has significant effects on elastic wave propagation. Although the characterization of partially frozen systems using velocity information has been well developed, application of the attenuation information is limited because the attenuation mechanisms in partially frozen systems are poorly understood. We have conducted ultrasonic wave transmission measurements with changing temperatures from 0°C to [Formula: see text] to estimate the effect of partially frozen liquids grown in unconsolidated (unconsolidated sediment) and consolidated (synthetic porous rock) materials on the velocity and attenuation of P- and S-waves. Our experimental results determined that the existence of partially frozen liquid in the unconsolidated and consolidated materials increases the velocity and attenuation for temperatures of 0°C to around the freezing point (i.e., [Formula: see text]), thus experimentally validating the unintuitive observations of high velocity and high attenuation. We interpreted the differences in velocity-versus-temperature curves as both the difference in inherent stiffness between the matrix of the consolidated material and the ice frame of the partially frozen unconsolidated material and the microscale ice distribution in pore spaces. We have also attributed the difference in the attenuation-versus-temperature curves in the unconsolidated and consolidated materials between the P- and S-waves to the difference of attenuation mechanism between the P- and S-waves. Our findings can be used for interpreting the velocity and attenuation results from the sonic logging measurements.

Transforming car glass into microphones using piezoelectric transducers

The use of car windows as a long range acoustic sensing device for external alarm signals is described in this paper. The goal is to detect and localize siren signals (e.g. ambulances and police cars) and to alert presbycusic drivers of its presence by visual and acoustic feedback in order to improve individual mobility and increase the sense of security. The glass panes of a Renault Zoe operating as an acoustic antenna have been equipped with large 50 mm outer diameter piezoceramic rings, hidden in the lower part of the door structure and the lower part of the windshield and the rear window. The response of the glass to a sweep excitation has been recorded. In general, the glass pane is acting as a high pass filter due to its inherent stiffness and provides only little damping. This effect is compensated by using a charge amplifier electronic circuit. The detection capability up to 120 m in static conditions as well as the influence of wind and vibration during driving conditions is shown. Finally, two alarm detection scenarios are reported with the car passing by a static alarm source once facing the car and once orthogonally to the driving direction.

OPTIMIZED LOW-EMBODIED AND OPERATIONAL CARBON SOLUTIONS FOR SINGLE-STORY INDUSTRIAL BUILDINGS

Detailed studies have been performed with the aim of determining optimum low-carbon solutions for buildings, and investigating the complex issues involved in their delivery. The evidence presented below suggests that building envelope specification has reached the point where the embodied carbon of any additional insulation balances, and may even outweigh, the corresponding savings in operational carbon. However, the extra material in the envelope has an inherent strength and stiffness that could be utilized to reduce the embodied carbon in the structure if appropriately designed. An extensive series of analyses was undertaken to (a) quantify the aggregated operational and embodied carbon related to modern envelope systems, and (b) evaluate the opportunities for embodied carbon reduction of the frame through the exploitation of the envelope’s structural capability. Particular attention was given to the use of long-span composite panels to reduce the number of supporting structural members. It was found that a considerable saving in embodied carbon is possible compared to traditional construction solutions. The study also suggested the absolute significance of combining operational and embodied carbon analyses, in order to demonstrate the effectiveness of carbon reduction strategies and requirements to shift away from “operational carbon only” methods. The focus of the initial phase of the work has been single-story industrial buildings, but the conclusions are applicable more broadly.

Linear stiffness compensation using magnetic effect to improve electro-mechanical coupling for piezoelectric energy harvesting

Piezoelectric transducers have been widely used in vibration-based energy harvesting. The efficiency of piezoelectric energy harvesting depends to a large extent on the electro-mechanical coupling of the harvester. Improvement of the coupling effect is possible through a variety of means from the transducer material-level to the device-level. At the device-level, the coupling effect of a cantilever-type energy harvester is inversely proportional to its mechanical stiffness which consists of the stiffness of the cantilever beam and that of the piezoelectric transducer. The mechanical stiffness, however, cannot be easily reduced because of the necessary inclusion of the cantilever beam and the inherent stiffness of the transducer. Both the beam and the transducer cannot be made arbitrarily thin due to typical design considerations. This paper reports a device-level approach for improvement of energy conversion efficiency by directly compensating the effective stiffness of a cantilever-type vibration energy harvester through magnetic effect. Specifically, two sheet-type permanent magnets are placed in the vicinity of the piezoelectric composite beam, and another slender magnetic bar is attached onto the beam together with an iron proof mass where one of its poles is located between the two magnetic sheets. It is shown analytically and experimentally that, under such configuration, a linear magnetic field yielding linear force pointing toward the magnetic sheets can be produced, which results in the reduction of the effective stiffness of the energy harvester to improve the electro-mechanical coupling. The experimental case studies demonstrate that the electro-mechanical coupling coefficient can be increased by 65% with 44.1% stiffness compensated. Both the open-circuit voltage and the power output are enhanced.

Persistent draining crossover in DNA and other semi-flexible polymers: Evidence from hydrodynamic models and extensive measurements on DNA solutions.

Although the scaling theory of polymer solutions has had many successes, this type of argument is deficient when applied to hydrodynamic solution properties. Since the foundation of polymer science, it has been appreciated that measurements of polymer size from diffusivity, sedimentation, and solution viscosity reflect a convolution of effects relating to polymer geometry and the strength of the hydrodynamic interactions within the polymer coil, i.e., "draining." Specifically, when polymers are expanded either by self-excluded volume interactions or inherent chain stiffness, the hydrodynamic interactions within the coil become weaker. This means there is no general relationship between static and hydrodynamic size measurements, e.g., the radius of gyration and the hydrodynamic radius. We study this problem by examining the hydrodynamic properties of duplex DNA in solution over a wide range of molecular masses both by hydrodynamic modeling using a numerical path-integration method and by comparing with extensive experimental observations. We also considered how excluded volume interactions influence the solution properties of DNA and confirm that excluded volume interactions are rather weak in duplex DNA in solution so that the simple worm-like chain model without excluded volume gives a good leading-order description of DNA for molar masses up to 10(7) or 10(8) g/mol or contour lengths between 5 μm and 50 μm. Since draining must also depend on the detailed chain monomer structure, future work aiming to characterize polymers in solution through hydrodynamic measurements will have to more carefully consider the relation between chain molecular structure and hydrodynamic solution properties. In particular, scaling theory is inadequate for quantitative polymer characterization.

Design of a simple, lightweight, passive-elastic ankle exoskeleton supporting ankle joint stiffness.

In this study, a passive-elastic ankle exoskeleton (PEAX) with a one-way clutch mechanism was developed and then pilot-tested with vertical jumping to determine whether the PEAX is sufficiently lightweight and comfortable to be used in further biomechanical studies. The PEAX was designed to supplement the function of the Achilles tendon and ligaments as they passively support the ankle torque with their inherent stiffness. The main frame of the PEAX consists of upper and lower parts connected to each other by tension springs (N = 3) and lubricated hinge joints. The upper part has an offset angle of 5° with respect to the vertical line when the springs are in their resting state. Each spring has a slack length of 8 cm and connects the upper part to the tailrod of the lower part in the neutral position. The tailrod freely rotates with low friction but has a limited range of motion due to the stop pin working as a one-way clutch. Because of the one-way clutch system, the tension springs store the elastic energy only due to an ankle dorsiflexion when triggered by the stop pin. This clutch mechanism also has the advantage of preventing any inconvenience during ankle plantarflexion because it does not limit the ankle joint motion during the plantarflexion phase. In pilot jumping tests, all of the subjects reported that the PEAX was comfortable for jumping due to its lightweight (approximately 1 kg) and compact (firmly integrated with shoes) design, and subjects were able to nearly reach their maximum vertical jump heights while wearing the PEAX. During the countermovement jump, elastic energy was stored during dorsiflexion by spring extension and released during plantarflexion by spring restoration, indicating that the passive spring torque (i.e., supportive torque) generated by the ankle exoskeleton partially supported the ankle joint torque throughout the process.

Vibration isolation using six degree-of-freedom quasi-zero stiffness magnetic levitation

In laboratories and high-tech manufacturing applications, passive vibration isolators are often used to isolate vibration sensitive equipment from ground-borne vibrations. However, in traditional passive isolation devices, where the payload weight is supported by elastic structures with finite stiffness, a design trade-off between the load capacity and the vibration isolation performance is unavoidable. Low stiffness springs are often required to achieve vibration isolation, whilst high stiffness is desired for supporting payload weight. In this paper, a novel design of a six degree of freedom (six-dof) vibration isolator is presented, as well as the control algorithms necessary for stabilising the passively unstable maglev system. The system applies magnetic levitation as the payload support mechanism, which realises inherent quasi-zero stiffness levitation in the vertical direction, and zero stiffness in the other five dofs. While providing near zero stiffness in multiple dofs, the design is also able to generate static magnetic forces to support the payload weight. This negates the trade-off between load capacity and vibration isolation that often exists in traditional isolator designs. The paper firstly presents the novel design concept of the isolator and associated theories, followed by the mechanical and control system designs. Experimental results are then presented to demonstrate the vibration isolation performance of the proposed system in all six directions.

Cyclic behavior of braced concrete frames: Experimental investigation and numerical simulation

RC shear walls have been widely used as the main lateral-load resisting system in medium and high-rise buildings because of their inherent large lateral stiffness and load resistance. But, in general, the energy dissipating capacity of RC shear walls is not very good and it is found that using the bracing system gives good results. The main purpose of this paper is to study the effect of the different types of bracing on the lateral load capacity of the frame. Also, the research contains a comparison between the braced and infilled frames to decide the best system. The research scheme consists of four frames; the bare frame, two frames one was braced with concrete, the second was braced with steel bracing and the fourth frame was infilled with solid cement bricks. All the specimens were tested under cyclic loading. The results gave some important conclusions as; braced and infilled the bare frames increased the lateral strength of the bare frame depending on the type of bracing and infill. Also, the different types of bracing and the infill increased the initial stiffness of the bare frame by a reasonable value. The energy dissipation for the braced and infilled frames is always higher than that for the bare frame up to failure. Also, numerical modeling was carried out with the nonlinear software platform (IDARC). The numerical results obtained with the calibrated nonlinear model are presented and compared with the experimental results. Good agreement was achieved between the numerical simulation and the test results.

Vibration isolation using a hybrid lever-type isolation system with an X-shape supporting structure

This study presents some novel results about analysis and design of low-frequency or broadband-frequency vibration isolation using a hybrid lever-type isolation system with an X-shape supporting structure in passive or semi-active control manners. It is shown that the system has inherent nonlinear stiffness and damping properties due to structure geometrical nonlinearity. Theoretical analysis reveals that the hybrid isolation system can achieve very good ultra-low-frequency isolation through a significantly-improved anti-resonance frequency band (by designing structure parameters). Noticeably, the system can realize a uniformly-low broadband vibration transmissibility, which has never been reported before. Cases studies show that the system can work very well with good isolation performance subject to multi-tone and random excitations. The results provide a new innovative approach to passive or semi-active vibration control (e.g., via a simple linear stiffness control) for many engineering problems with better ultra-low/broadband-frequency vibration suppression.

Adjacent Equilibria in Highly Flexible Upright Loop on Rigid Foundation

For very slender structural components, self-weight may compete with elastic flexural stiffness in determining equilibrium configurations. In cases where the inherent elastic stiffness is low (relative to self-weight) we observe a variety of types of highly nonlinear behavior in the equilibrium shapes, together with changes in the natural frequencies of small oscillations about these equilibrium configurations. This technical note describes a specific phenomenon observed in experiments on very slender polycarbonate loops. In addition to profound changes in equilibrium shapes as a function of weight-to-stiffness ratio, under some circumstances it is possible to have two adjacent, co-existing equilibrium configurations. This robust, highly nonlinear snap-through behavior is demonstrated by perturbing from one shape to the other.

Cyclic Behavior of Braced Concrete Frames: Experimental Investigation and Numerical Simulation

Abstract RC shear walls have been widely used as the main lateral-load resisting system in medium and high-rise buildings because of their inherent large lateral stiffness and load resistance. But, in general, the energy dissipating capacity of RC shear walls is not very good and it has been found that using the bracing system gives good results. The main purpose of this paper is to study the effect of different types of bracing on the lateral load capacity of the frame. Also, the research contains a comparison between the braced and infilled frames to decide on the best system. The research scheme consists of four frames; the bare frame, two frames the first of which was braced with concrete, the second was braced with steel bracing and the fourth frame was infilled with solid cement bricks. All the specimens were tested under cyclic loading. The results gave some important conclusions; braced and infilled bare frames increased the lateral strength of the bare frame depending on the type of bracing and infill. Also, the different types of bracing and the infill increased the initial stiffness of the bare frame by a reasonable value. The energy dissipation for the braced and infilled frames is always higher than that for the bare frame up to failure. Also, numerical modeling was carried out with the nonlinear software platform (IDARC). The numerical results obtained with the calibrated nonlinear model are presented and compared with the experimental results. Good agreement was achieved between the numerical simulation and the test results.

The Role of Cellular Mechanical Properties in Microenvironment-Dependent Behavior

Understanding the role of mechanics in cell behavior is necessary for effective development of tissue engineering therapies. This study observed how cellular mechanical phenotype influenced cell morphology and actin organization on substrates coated with different extracellular matrix ligands. Inherent cell stiffness, represented by Young's modulus, for MG-63 (osteosarcoma) and SH-SY5Y (neuroblastoma) cells was mechanically determined using atomic force microscopy. These cells have dissimilar moduli when probed over the nucleus (1.3±0.5 and 0.26±0.15 kPa for MG-63 and SH-SY5Y cells, respectively). To investigate the effect of substrate compliance, cells were cultured on polyacrylamide gels ranging from 0.3 to 12 kPa and functionalized with fibronectin, laminin, or collagen-1. We hypothesized that cells would spread and attach more successfully to substrates with an elastic modulus equal to or greater than that of the cell, provided physiologically relevant ligands are present. Cellular responses were captured daily over 4 days using fluorescent microscopy. MG-63 cells showed two different organizational phenotypes: a multi-cell, spheroid-like structure and a spread monolayer. The threshold substrate compliance required for MG-63 cells to organize into either phenotype was designated by the cells’ own stiffness, most clearly observed on collagen-1 coated gels. While SH-SY5Y cells exhibited similar phenotypic organizations when plated on varying substrates, no mechanical compliance threshold was observed. However, SH-SY5Y cells exhibited greater tendencies toward forming spheroids when plated on fibronectin. These morphological changes show a possible correlation between the cells’ characteristic mechanical phenotypes and their relationship to the microenvironment. Cells exhibiting less compliant phenotypes may have more substrate stiffness-dependent behavior, whereas more compliant cell types may have more ligand-dependent behavior. These findings indicate that cellular mechanosensing is influenced not only by the mechanical and biological characteristics of the microenvironment but also by the inherent mechanical phenotype of cells comprising a population.

Plasticizing effects of epoxidized sun flower oil on biodegradable polylactide films: A comparative study

Polylactide (PLA) is one of the most promising materials among the renewable source-based biodegradable plastics. However, high inherent stiffness and brittleness of the pure PLA is often insufficient for wide range of engineering applications. One of the best ways to improve the processability, toughness and flexibility of PLA is to plasticize with epoxidized plant oils. In this work, epoxidized sun flower oil (ESFO) was incorporated into PLA matrix. The thermal, mechanical, biodegradation, optical transmission properties and fracture morphology of ESFO plasticized PLA were investigated to make a comparison with that of PLA plasticized by commercial epoxidized soya bean oil (ESO). Results show that a remarkable improvement of elongation at break was observed in the case of ESFO incorporated PLA. Although a slightly decrease the T g of PLA was resulted from the plasticizing effects of ESFO, the thermal stability of the plasticized PLA was improved. On the other hand, the ESFO plasticized PLA showed a higher level of UV adsorption but a lower level of biodegradation ratio. After all, ESFO exhibited similar effects on the biodegradable PLA films to ESO, which is anticipated to be a good candidate for plasticizing biodegradable polymer materials.

Toxicity assessment of a novel silk fibroin and poly-methyl-methacrylate composite material

Based on its biocompatibility, silk fibroin has been applied in the fabrication of biomaterials in various forms such as gels, powders, fibers, and membranes. Poly-methyl-methacrylate (PMMA), which has been successfully used as a filler material in vertebroplasty, has the disadvantage of excessive inherent stiffness, which may damage adjacent vertebral bodies in elderly patients with osteoporosis. A novel potential filler material, consisting of silk fibroin and PMMA (silk/PMMA), was tested to assess its cytotoxicity, genotoxicity, and intramuscular implantation experiment according to ISO10993. The MTT assay, Ames test, mouse lymphoma assay, and intramuscular implantation were performed using extracts from solidified silk/PMMA, according to the protocol in ISO10993. Extracts of silk/PMMA did not show cytotoxicity, genotoxicity or toxic reaction in intramuscular implantation. These findings suggest that the silk/PMMA composite materials may potentially be used as a non-toxic biomaterial.

The Differences between Section Rigidity and Member Stiffness and their Applications in Engineering Structural Design

Under the action of same loads, the sizes of deformation and internal force generated in the building structure are related to the inherent stiffness of member or structure itself. In structure design, these two basic concepts of section rigidity and member stiffness play a significant role in stress analysis of member or structure. This article summarizes the application of section rigidity and member stiffness which are often used in structural design by contrasting the definition and calculation of section rigidity with those of member stiffness, which can help the designers to understand the two concept of “rigidity” well and utilize them reasonably in real structure designs, and make the designed structural force conform to relative codes and regulations.